1. Field of the Invention
The present invention relates to a positive electrode for a lithium secondary battery using lithium as the negative active material. Such a battery is useful as a small, lightweight power source in portable electronic appliances and electric vehicles because of its high voltage and high capacity, ease of manufacture, and a nonaqueous electrolyte lithium secondary battery employing the same positive electrode.
2. Description of the Prior Art
In order to produce batteries having a high performance, a small size and a light weight for use in portable appliances and capable of increasing the mileage of electric vehicles, new battery systems of smaller size and lighter weight are being intensively researched and developed throughout the world. Among them, recently, the lithium secondary batteries which have the properties of high voltage and high capacity and are lighter than other new battery systems. The lithium secondary battery is a common name for various battery systems employing lithium as the negative active material. The negative electrode is available in various forms as follows.
In the simplest form, the metal lithium is itself used as the negative electrode. This results in a battery system with very high voltage, but the life of the battery is limited with regard to cycles of charging and discharging. To circumvent this problem, various negative electrodes mainly composed of metals in which lithium is disposed and alloyed easily at normal temperature, such as aluminum and aluminum alloys including Al-Mn system, alloys composed of lead, tin, bismuth, cadmium and the like represented by Wood's metal, or lithium active material holder in which lithium repeats doping and undoping, such as polyaniline, polypyrrole, other conductive polymers, certain oxides, sulfides, and carbons are used.
In contrast, as the positive active material containing oxides of many transition metals such a MnO.sub.2, TiS.sub.2, MoS.sub.2, Nb.sub.2 O.sub.5, Cr.sub.2 O.sub.5, LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2, LiMnO.sub.2, and LiMn.sub.2 O.sub.4, chalcogen compounds, and double oxides with lithium have been studied. These compounds have a layer or tunnel crystal structure like intercalation compounds, and contain substances which permit lithium ions to repeat undoping and doping by charging and discharging. Primarily, it is LiCoO.sub.2, LiNiO.sub.2, and LiMn.sub.2 O.sub.4 which are not used as the positive active material in nonaqueous electrolyte lithium secondary batteries with high voltage in the 4 V class. These are known as lithium ion storage batteries. In particular, the double oxide of lithium and cobalt, LiCoO.sub.2, has a high voltage and capacity, and has been shown to be practical because of its excellent properties such as superior rechargeability. However, cobalt, which is the principal material in these batteries, is relatively expensive, its production is geographically limited. Hence, fluctuation in supply and price due to political circumstances are anticipated.
On the other hand, in double oxides of lithium and manganese or nickel such as LiMn.sub.2 O.sub.4 or LiNiO.sub.2, the manganese or nickel component is relatively inexpensive, and the supply is stable, but the characteristics, in particular, rechargeability are inferior to those of LiCoO.sub.2. Therefore, attempts to modify these compounds by investigating methods of synthesis and/or treatments has been undertaken. In particular, LiNiO.sub.2 has a similar crystal structure as LiCoO.sub.2, although the potential of LiNiO.sub.2 is about 0.2 V lower. Attempts have been made to use it as a substitute for LiCoO.sub.2 by increasing its capacity and equalizing the energy density.
Methods of synthesis of double oxides of lithium and nickel, such as LiNiO.sub.2, are known in the art. Representative compounds are found in the literature as follows. (1) J. American Chemical Soc., Vol. 76, p. 1499 discloses anhydrous lithium hydroxide and metal nickel which are mixed, and heated to react with atmospheric oxygen to generate LiNiO.sub.2. When used as the positive active material in a lithium secondary battery, the discharge capacity was low, and the properties were inferior. (2) Chemistry Express, Vol. 6, No. 3, 161, 1991 discloses an aqueous solution of equimolar 4.5 mol/L lithium hydroxide and an aqueous solution of 1.0 mol/L nickel nitrate which are mixed at 60.degree. C., the mixture is stirred for a long time at the same temperature, then filtered, decompressed, dried and solidified to obtain a precursor. This precursor is ground to a powder, which is preburned at 300.degree. C. and then burned at 800.degree. C., and a double oxide of lithium and nickel is obtained. It is reported that this double oxide may be used as a positive active material with relatively high capacity.
Thus, if the method of synthesis of the double oxide is simple, the properties of LiNiO.sub.2 as positive active material were inferior. On the other hand, positive active materials with excellent properties require a complicated process for their synthesis involving many steps, a long time, and control of the process was difficult when manufacturing large quantities industrially, i.e. not in the laboratory, and finally, the repeatability of rechargeability is poor.